Automatic self-compensating magnetic compass system



AUTOMATIC SELF COMPENSATING MAGNETIC COMPASS SYSTEM Original Filed Se t.29, 1942 2 SheetsSheet 1 NORTH Wilfrid G.WH1' T.E.-

INVENTOR I WW ATTORNEY Jan. 29, 1946. w. G. WHITE 2,393,670

' AUTOMATIC SELF GOMPENSATING MAGNETIC COMPASS SYSTEM Original FiledSept. 29, 1942 2 Sheet-Sheet 2 Wilfrid [-3. Whfle INVENTOR ATTORNEY uPatented Jan. 29, 1946 AUTOMATIC SELF-COMPENSATING MAG- NET IC COMPASSSYSTEM Wilfrid Gordon White, United States Navy Continuation ofapplication Serial No. 460,113, 1 September 29, 1942. This applicationMarch 18,

p 1943, Serial No. 479,590

10 Claims. (Cl. 33-225) (Granted under the act of March 3, 1883, as

amended April 30, 1928: 370 0. G. 757) This inventlonrelates tocompensating systems, and particularly to a compensating system forvehicle course determining devices, especially magnetic compasses and isa continuation of my application, Serial No. 460,113 filed September Itis a'primary object of this invention to provide a repeater system whichwill indicate correct magnetic directions, or more specifically, a

repeater system which will automatically compensate for the magneticcompass deviations which are caused by the metallic parts of the vehicleupon which the compass is mounted and other localized metal,particularly ferric metal which may comprise the vehicle's cargo.

Other objects and advantages of this invention will appear more fullyhereinafter from the following description taken together with theaccompanying drawings which illustrate two embodlments of the invention.It is to be expressly understood however, that the drawings are for thepurpose of illustration only and not for the purpose of limitation,reference being had for this latter purpose to the appended claims.

In the drawings:

Fig. 1 illustrates in elevation a vessel upon which are shown, inexaggerated size, two magnetic compasses mounted in position relative tothe horizontal iron comprising the vessel's cargo;

Fig. 2 illustrates diagrammatically the deviation produced by thehorizontal iron;

Fig. 3 is a Napiers diagram of the deviations of the compasses shown inFig. 1;

Fig. 4 is a diagrammatic view showing one arrsngement of apparatusillustrative of one embodiment of this invention; and

Fig. 5 is adiagrammatic view showing a modification of Fig. 4 adapted torepeat compass directions through 360".

This invention is predicated upon the-discoveryby the applicant that twomagnetic compasses may be positioned relative to each other and to thelocalized ferric-metal so that the deviations produced in each by thesaid localized metal will be of the same sense but of a different order.The deviation of the compass which is positioned at the greater distancefrom the disturbing influence of the localized ferric metal will be lessthan the deviation of the compass which is positioned nearer the saidmetal. The deviation of the compass located at the greater distance willbe equal to a determinable factor or ratio multiplied by the differencein the deviation of both compasses, or the difierence in the vehicle'sheadings as indicated by each compass. When the compasses are positionedso as tocomply with the conditions stipulated, it is a relatively sim-Dle matter to finjd the compass deviation on all headings and in anylatitude.

For purposes of illustrating this invention, there is shown in Fig. l asteering and standard compass I0 and II, respectively, each mounted on awooden ship II which has no magnetic qualities. It is assumed forpurposes of illustration, that the deviation in eachof the Symmetricallypositioned compasses l0 and I2 is caused solely by the local horizontaliron I3 which is assumed to comprise the vessels cargo. This horizontaliron l3 as shown in Fig. 2 is symmetrically placed relative to the foreand aft axes of the vessel and to the center of the compass since it isthe usual practice to position the compass so that substantially all ofthe horizontal iron is symmetrically placed relative thereto. Thissymmetrically placed horizontal iron causes a deviation which isclassified according to Navy standard usage as quadrantal" deviationsince it changes in sign in each successive quadrant as the ship's headswings in azimuth and has its maximum and minimum value in eachquadrant.

The horizontal iron i3 causes the lines of force of the earths field, intaking the path of lowest reluctance, to crowd through the iron so as tofollow the direction of its length. When the ship is heading northmagnetic, these lines of force follow the direction of the length ofiron which also has its axis on the magnetic meridian. The compass showsno deviation for the heading of magnetic north. When the ships head isnortheast, magnetic, as illustrated in Fig. 2, the lines of force of theearths field are drawn out of their usual direction toward the directionof the length of iron or along its axis. The compass will thus have aneasterly deviation which is maximum on this heading.

Fig. 2, which shows the ship II on a northeasterly heading, also showsthe lines of force of the earth's magnetic field crowdedinto thehorizontal iron l3 thus illustrating the distortion of the earths fieldwhich produces the maximum easterly deviation. In Fig. 3 there is showna plot of the deviation of both the standard and steering compasses on aNapier's diagram. This diagram is useful primarily in applying correctlythe errors produced by the local iron, but is set forth here merely as aconvenient means of illus trating the deviation of both compasses on allships headings from 0 north magnetic, easterly to south magnetic. Theother half of the diagram for headings varying from 180 westerly to 360is not shown since it would be but a duplication under the assumedconditions of the two quadrants shown in Fig. 3. It is important to notefrom the diagram that differences in the ships headings as indicated onthe diagram can be used to determine the deviation of the standardcompass. For example, assume that the uncorrected steering compassindicated a ships heading of 15 east. The compass deviation on thisheading as obtained from the diagram in Fig. 3 is 30 east, and themagnetic heading corresponding to the indicated heading would be 45east. In order to ascertain from the diagram the heading of the ship asindicated by the standard compass for a magnetic heading of 45 east,departure along the solid line shows a deviation of 6 east. The headingper standard compass would (following the dotted line) be 39 east. Thedifference in the heading of the ship as indicated by the two compassesis .24 east and the fixed ratio or factor for the assumed condition isone-fourth; consequently, one-fourth of the difference in indicatedheadings will for all headings equal the deviation of the standardcompass.

Since it is apparent from the foregoing and particularly from aninspection of Fig. 3 that the deviation of the standard compass willalways be one-fourth of the difference in the heading indicated by boththe standard and steering compass, it remains but to provide a system,and preferably an electrical repeating system, which will automaticallyapply a correction to the instrument repeating the ships heading asindicated by the standard compass. The applied correction is obtained inthis embodiment of the invention by taking one-fourth of the differencein the electrical output of each electrical transmitter associated withthe standard and steering compasses and applying this electricalquantity to the repeating instrument associated with the said standardcompass.

The repeating system of the prior art, especially those operating uponthe principle of the Wheatstone bridge, have been selected to exemplifythis invention. As a specific embodiment thereof, the repeating systemsillustrated and described in the United States patent to F. West, Jr.,No. 2,111,442, issued on March 15, 1938,

has been selected. This particular telemetering system is not adapted torepeat compass directions throughout 360 and is selected merely becauseit affords a convenient and simplified means of illustrating thisinvention.

In Fig. 4 there is shown both the standard and steering compasses l andI2 as having an arcuate condenser plate l6 and I6 each convenientlysecured to the pivoted compass card l4 and I4. These compass cards areactuated by the magnetic elements represented for this purpose at l3 andI3. As illustrated, the standard compass indicates a ships head of 39east; whereas the steering compass indicates a ships head of east. Theplate l6 movable with the magnetic element of the standard compass isconnected by means of the conductor 26 to one output terminal of asource of high frequency alternating current of a substantially constantamplitude. For this purpose there is illustrated an L. C. oscillatorcircuit which includes the usual triode tube 2|, inductively coupledcoils 22, grid leak 23, by-pass condenser 24 and a source of platecurrent 25. It may be desirable to provide an amplifier for raising theamplitude output of the oscillator to a predetermined maximum level inwhich event those amplifiers having substantially undistorted amplitudeoutput should be used or a constant amplitude control circuit for theamplified output of the oscillator hould be provided.

A pair of condenser plates l1 and I8 are positioned about the peripheryof the compass card |4 so as to be variable capacitively coupled withthe plate I6 as the plates I1 and I8 move about the plate I6 whichchanges in the ships heading. Each plate I! and I8 is connected throughthe conductors 30 and 3|, respectively, and the diode tubes 32 and 33 toa return line leading to the oscillator circuit 20. Thus, a lowimpedance return path is established for alternate half cycles of thecurrent flowing in the paths established by the coupling betweencondensers |6-|'| and |6|8. The plates l1 and I8 are also connected tothe potentiometer 39 through the coils 31 and 38 by means of theconductors 34 and 35 to form two arms of the capacity impedance branchof the Wheatstone bridge 36. The return line 4| leading from thepotentiometer to the oscillator divides the potentiometer so that itforms the two remaining arms of the impedance capacitive bridge andprovides means for balancing the bridge on 0 headings.

Means is thus provided for rectifying the alternating current in thebranch lines 34 and 35 of the bridge network so that the winding 42 of adirect current center zero meter such as that disclosed at 40 may beconnected to the output diagonals of the impedance capacitive bridge 36.

The condenser plate l6 of the steering compass ID is also secured to thecompass card |4 so as to be actuated by the magnetic element I3supported thereon and is connected by means of the conductor 26' to ahigh frequency alternating supply circuit 20'. The arcuate condenserplates I1 and I8 are each secured to the compass bowl so as to bemovable with the ship as it swings in azimuth to vary the couplingbetween these condenser plates. The plates I1 and I8 are each connectedby means of conductors 30' and 3| and diode tubes 32' and 33respectively to the return line leading to the supply circuit 20' toprovide a low impedance path shunting altemate half cycles of the supplysource current. The plates I1 and I8 are also connected through thechoke coils 31 and 33 to the potentiometer 39' to form two arms of thecapacitive impedance branch of the Wheatstone circuit 38'. Thepotentiometer 39 is connected to the return line of the oscillator 20 bythe conductor 4|. The output voltage of the bridge networks 36 and 36'is connected across potentiometer 44 so that the potential drop acrossthe potentiometer 44 will equal the difference between the outputs ofthe two bridge circuits. It is, therefore, apparent that means isprovided for obtaining a direct current voltage which varies linearlywith changes in headings as indicated by the separate compasses l0 andI2; and also that means is provided for obtaining a voltage equal to thedifference in output voltages which represent the headings of eachcompass.

The center tap 45 of the potentiometer 44 is set at a position so thatthe voltage across the leads 46 and 41 is equal to the difference involtage output of each bridge network multiplied by the predeterminedfactor, and under the assumed condition is set to a position wherein thevoltage across the leads 46 and 41 is equal to one-fourth of the voltagedrop in the potentiometer 44. The

leads 46 and 41 are connected to the compensating coil 43 of the directcurrent instrument 40. Thus the indicator P of the instrument 40 willnot repeat the position of the compass I2 but will rather indicate thecorrect magnetic heading of the ship corresponding to the headingindicated by the compass I2.

As a more practical application of the invention, the use of atelemetering system illustrated in the patent to F. West, Jr., No.2,277,027, is preferred since with the system therein disclosed, thecompass direction may be repeated throughout 360. Fig. 5 illustrates themodification of my invention wherein the standard and steering compassesI2 and I each have an arcuate condenser plate H3 and I I3 mounted ontheir respective compass cards III and II I for simultaneous movementtherewith. These condenser plates H3 and H3 as illustrated have anarcuate extension of substantially 90 and are each connected by theconductors I26 and I26 to one terminal of a separate high frequencyalternating current such as the oscillator circuits I20 and I20. Theseplates each form the movable plate of a variable condenser which alsoincludes four separate similarly arcuated condenser plates H4, H5, H6,and I I1, each supported within thebowl of the compass I2 closelyadjacent the path of movement of the movable plate I I3. Likewise, thefour plates H4, H5, H6 and III are secured to the inside of the bowl ofthe compass I0 to lie closely adjacent the paths of movement ofthe'movable plate H3. All of the plates are perfectly concentric withthe compass cards so that all points on the condenser plates H3 and H3are always equal distance from the opposite points on theircorresponding stationary plates H4--H'I and II4'--II'I of the compassesI2 and I0, respectively.

Each diametricall opposed pair of stationary condenser plates, forexample, plates H4 and I I6, are connected to the potentiometer I39through the choke coils I36 and I3! by means of the leads I30 and I3I.The adjustable element I38 of the potentiometer I39 is connected to forma ground connection with the supply source I20. The rectifier tube I34is connected to the leads I 30 and I3I to provide a low impedance pathto ground for alternate half cycles of the alternating current flowingin the leads I30 and I3I. The leads I32 and I33 connect the coil I4I ofthe repeating instrument I40 with the lines leading from the oppositediagonals of the impedance capacitive bridge I35 formed by the condenserplates H3, I I4 and H6 and the potentiometer I39. The dia-' metricallyopposed plates H5 and H! are also connected by the leads I30a and I3Iathrough the choke coils I36a and I3Ia to opposite terminals of apotentiometer I33a which also has its adjustable tap I38a connected to aground connection with the alternating current supply source to form animpedance capacity bridge I35a. The full wave rectifier tube I34a isconnected to provide a low impedance path to ground for alternate halfcycles of the alternating current flowing in the leads I30a and I3Ia.The output of this bridge I35a is connected by means of the leads I32aand I33a to the coil I42 of the repeating instrument I40.

A similar arrangement is provided for each pair of diametrically opposedcondenser plates H4 and H6 and H5 and H1 of the compass I0 which areconnected through lines I30 and I3I' and I30"and HI and choke coils I36and I31 and I36 and I31, respectively, to the opposite terminals of thepotentiometer I39 and I39".

Each potentiometer has a. movable tap connection I38 and I33 to ground.It is thus apparent that diametrically opposite condenser plates eachform the arms of one branch of separate impedance capacitive bridgecircuits indicated generally at I35 and I 35". In order that directcurrent may be obtained in the coils HI and I42 to actuate the movablevane I45 of the repeating instrument I40, the double anode tubes I34 andI34" are connected in circuit to provide a low impedance groundconnection with the alternating current supply source I20 and I20 foralternate half cycles of the current flowing in lead lines I30-I3I andI30I3I". Adirect current potential is thus established in each separatebridge circuit and differences in the relative capacity of the bridgecircuit resulting from movement of the condenser plates H4I I1 and II4-II'I' relative to the condenser plate H3 and H3, respectively, willresult in corresponding difierences of potential being establishedacross the output terminals of each bridge circuit. The polarity of thevoltage across the bridge circuits output terminals is dependent uponthe coupling between diametrically opposite condensers and as themovable condenser plate is moved from a position adjacent one stationaryplate to a position adjacent a diametrically opposite plate the polarityof the bridge circuit output voltage is changed.

The output voltage of the bridge circuits I35 and I 35 are eachimpressed across a potentiometer I50 so that their voltage opposes eachother. The adjustable contact I52 of the potentiometer is set at aposition wherein the voltage across the leads I54 and I55 will be equalto the difference in voltage output of the bridge circuit I36 and I35multiplied by the predetermined ratio. These leads are connected to thecompensating coil I4I which is concentric with the coil I. the outputvoltage of the bridge circuits mm and I36 are each impressed across thepotentiometer I5I so that their voltages oppose each other. Theadjustable contact I53 is set at a position wherein the voltageimpressed across the leads I50 and I5! will be equal to the differencein output voltage of the bridge circuits I 35a and I35 multiplied by thefixed predetermined ratio. These leads are connected to the compensatorcoil I42 which is concentric with the main coil I42 of the repeatinginstrument I40.

As illustrated in Fig. 5, the compass bowl is positioned relative to thecompass card of the standard compass I2 to indicate a ships head of 39east in which position the plates H5 and H4 are moved relative to theplate I I3 so that the capacities of the separate bridge circuitswill beunequal and both bridges will be unbalanced. The capacitive couplingbetween plates H3 and H5 being greater than the capacitive couplingbetween plates H3 and M1, the voltage across the output terminals of thebridge circuit I35a will .be greater than that voltage which appearsacross the output diagonal of the bridge circuit I35. Thus, the voltageapplied to the coil I42 will be greater than the voltage applied to thecoil I. The resultant field produced by these two coils Likewise,

will, under the assumed conditions, when ener-- gized by the outputvoltage of their respective bridge circuits, have an axis along the lineformed at an angle of 39 relative to the plane of the coil I42. Themovable vane I45 would then take a position to indicate the heading ofthe standard compass I2.

The steering compass I0, as illustrated, indicates a ships heading of 15east, and since on this heading the coupling between movable and fixedplates is unequal the bridges I35 and I35" will be unbalanced, thevoltage output of the bridge circuit I35" will be greater than thevoltage output of the bridge circuit I35. Furthermore, the voltageappearing across the output diagonals of the bridge circuit I351: isunder the assumed conditons greater than the voltage which appearsacross the bridge circuit I35". Consequently, when one-fourth of thedifference between the outputs of these two bridge circuits is appliedto the compensating coil I42 of the repeating instrument I40, the fiuxproduced by the current flowing therein will be in the same direction asthe flux produced by the current flowing in the main coil I42 which isenergized by the voltage output of the bridge circuit I35a. But sincethe capacitive couplings between the plates H3 and I are less than thecapacitive couplings between the condensers H3 and N4, the voltage whichappears across the bridge diagonals I35 will be less than the voltageappearing across the bridge circuit I35. Therefore, when one-fourth ofthe difference between the outputs of the bridge circuits I35 and I35 isapplied to the compensating coil I4I' of the repeating instrument, thecurrent flowing therein will produce a fiux which is equal in magnitudebut opposite in direction to the flux produced by the current flowing inthe coil I4I. Thus, the movable vane I45 will assume a position alongthe axes of the concentric coils I42 and I42 to indicate a correctmagnetic heading of 45 east.

From observation of Fig. 3, it should be noted that in correcting thecompass, westerly errors in deviation must be subtracted from thecompass reading, whereas easterly errors are added. To obtain thecorrect magnetic headings for the compass readings it will be apparentthat voltages applied to the compensating coils HI and I42 must alwaysbe of a correct polarity. For example, assume that the standard compassI2 indicates a ships heading of 121 and the steering compass I indicatesa heading of 145. Under these conditons the voltage output of the bridgecircuit I35 would be greater than the voltage output of the bridgecircuit I35. However, the voltage output of the bridge circuit I35awould under these conditions be less than the voltage output of thebridge circuit I35". Consequently, the voltage applied to the coils I42and I42 would now produce opposing fluxes which are equal under theparticular conditions assumed.v The vane I45 will, therefore, take aposition along the axes of the coils MI and HI to indicate a correctmagnetic heading of 315.

It should be understood that the conditions under which the factor ofone-fourth is obtained are merely illustrative and may not occur inactual practice. The particular ratio used is dependent upon thelocation of the compasses relative to each other and to localized ferricmetals. Furthermore, a single source of high frequency alternatingvoltage may be utilized for energizing each repeating system rather thanresorting to separate sources as illustrated. It should also beunderstood that the entire disclosure is set forth merely forillustration and not for the purposes of limitation, reference for thislatter purpose being had to the subjoined claims.

This invention may be manufactured and used by or for the Government ofthe United States of America for governmental purposes without Ivariable in magnitude as a the payment, of any royalties thereon ortherefor.

I claim:

1. In a self compensating system, for indicating correct magneticdirections, the combination including two magnetic compasses spacedrelative to each other so that the deviation in each compass for anygiven heading will be of the same sense but of. a different order, firstmeans responsive to theldirectional indications, of said first compassfor producing a first quantity variable in magnitude as a function ofthe indicated direction of said first compass, second means responsiveto the directional indications of said second compass for producing asecond quantity function of the indiof said second compasawmeans cateddirection 4 for combining said) first and second quantities,

by said combinedquantities directional indication on any and meansoperable to produce a correct given heading. l

2. In a self compensatin system for indicating correct magneticdirections, the combination in cluding two magnetic compasses spacedrelative to each other so that the deviation in each for any givenheading will be of the same sense but of a different order, separatemeans operatively connected to be controlled by each compassrespectively for producing first and second quantities variable inmagnitude with the indicated headings of said compasses, meansoperatively connected to be controlled by said separate means forcontinuously producing a third quantity variable in magnitude with thedifference between said first and second quantities, means operativelyconnected to said last named means 'and said separate means producingsaid first quantity for combining said first andthird quantities toadjust said first quantity as a function of said difference, and meansresponsive to said first quantity as adjusted to indicate the correctmagnetic heading.

3. In a self compensating system for indicating correct magneticdirections, the combination including two magnetic compasses spacedrelative to each other so that the deviation in each for any givenheading will be of the same sense but of a different order, thedeviations of a first one being equal to a fixed ratio of the differencein their deviations, separate means operably connected to be controlledby each compass respectively for producing first and second quantitiesvariable in magnitude with the indicated headings of said compasses,means combining said quantities for producing a third quantity variableas the difference between said first and second quantities, means forcombining said first quantity with a portion of said third quantityequal to said fixed ratio, and means responsive to last said combinedquantities for indicating the correct magnetic heading.

4. In a self compensating system for indicat ing correct magneticdirections, the combination of two magnetic compasses spaced relative toeach other so that the deviations produced in each by local magneticmaterial will be of the same sense but of a different order for varyingheadings of the vehicle upon which the said compasses are mounted, thedeviations of the first being less than the deviation of the second andequal to a fixed determinable ratio of the difference in theirdeviations, separate means operable by the magnetic element of eachcompass for transmitting at a distance an electrical quantity having amagnitude equal to a function of the indicated direction of itsrespective compass, a repeating instrument operable by the electricalquantities having main and compensating coils, means connecting the maincoil of the repeating instrument to the transmitting means associatedwith said first compass so that the repeating instrument would normallyfaithfully repeat the indicated direction of said compass, means forderiving a differenceelectrical quantity equal to the difference inmagnitude of the electrical quantities transmitted by each of saidmeans, and means for applying to said compensating coils a portion ofsaid difference quantity equal to said fixed ratio, whereby theinstrument will indicate the correct magnetic headin corresponding tosaid compass heading.

5. In a self compensating system for indicating correct magneticdirections including in combination two magnetic compasses spacedrelative to each other so that the deviations produced in each by localmagnetic material on varying headings of the vehicle upon which they aremounted will be different, separate means for obtaining electricalquantities for reproducing at a distance the directional indications ofeach compass, each means inciudin impedance bridge circuits operativelyassociates with each compass, the electrical output of each impedancebridge circuit being variable in accordance with a function of saidindicated direction, a repeating instrument operable by said electricalquantities when energized by the output of the bridge circuitsassociated with each compass for faithfully repeating its indicateddirections, and means combining the electrical outputs of correspondingbridge circuits of each of said separate means to obtain a differenceelectrical quantity which when applied to said repeating instrumentenergized to repeat the indication of one of said compasses willcompensate for its deviation to indicate correct magnetic directionscorresponding to the directions indicated by said compass.

6. In a self compensating system for indicating correct magneticdirections, the combination including two magnetic compasses spacedrelative to each other so that the deviations produced in each by localmagnetic material will be of a different order but of the same sense,the deviation of one of said compasses being equal to a determinablefixed ratio of the difference in their deviations, separate meansoperably connected to be controlled by each compass respectively forobtaining an electrical quantity variable in magnitude in accordancewith changes in the indicated directio of each of said compasses, meansresponsive to said quantities for obtaining a third electrical quantityhaving a magnitude proportional to the fixed ratio of the difference insaid first two electrical quantities, and directional indicating meansoperable by the electrical quantity obtained from the means controlledby said first compass and said third electrical quantity for indicatingthe correct magnetic direction corresponding to the direction indicatedby said first compass.

7. In a self compensating system for indicating correct magneticdirections, the combination including two magnetic compasses spacedrelative to each other so that the deviation in each for any givenheading will be of the same sense but of a different order, thedeviations of a first one being equal to a fixed ratio of the differencein their deviations, means responsive to the directional indications ofsaid first compass for producing electrical quantities variable inmagnitude as a function of said indicated directions, means controlledjointly by said compasses for producing an electrical quantity whichvaries in magnitude as a function Of the changes in the difference ofsaid indicated headings of said first and second compasses, directionalindicating means including an indicator element and having at least amain winding and at least a compensating winding adapted when energizedto actuate said indicator element, means for energizing said mainwinding with the electrical quantity produced by said first named means,and means for energizing said compensating winding with a portion of theelectrical quantity produced by said second means equal to said quantitymultiplied by said fixed ratio.

a. A course maintaining device for vehicles comprising a first andsecond impedance bridge network, an electrical power supply source forenergizing each of said bridge networks, said first impedance bridgenetwork having an element movable by the earths magnetic field to alterthe impedance of the arms forming one branch of said bridge network,said second impedance bridge network having an element movable by theearths magnetic field to adjust the impedance of the arms forming onebranch thereof whereby the voltage appearing across the output diagonalsof each bridge network will vary in magnitude and direction as afunction of the position of their respective movable elements,- meanscombining the respective output voltages of the said bridge networks toproduce a difference voltage, means combining a portion of saiddifference voltage with the output voltage of one of said bridgenetworks to thereby compensate for the deviation in position of themovable element of last said bridge network produced by the distortionsin the sultingfrom localized ferric metal, and direction indicator meansresponsive to last said combined voltages.

9. In a self compensating system for indicating correct relativedirections, the combination including a pair of like directionindicating means each of which is subject to deviation, said indicatingmeans being spaced relative to each other so that the deviation in eachfor any given direction will be of the same sense but of a differentorder, the deviation of one being equal to a fixed ratio r f thedifference in their devia-- tions, separate means responsiverespectively to the direction indicated by each indicating means forproducing first and second quantities variable in magnitude as afunction of the direction indicated by each said indicatin means, meanscombining said first and second quantities for producing a thirdquantity variable as the difference between said first and secondquantities, means for combining said first quantity with a portion ofsaid third quantity equal to said fixed ratio, and means responsive tolast said com bined quantities for indicating the correct direction.

10. In a self-compensating system for indicating correct relativedirections, the combination including a pair of' like directionindicating means each of which is subject to deviation, said indicatingmeans being spaced relative to each other so that the'deviation in eachfor any given direction will be of the same sense but of a dif ferentorder, the deviation of one being equal to earths magnetic field reducing a third voltage variable as the difierence between said first andsecond voltages, means for combining said first voltage with a portionof said third voltage equal to said fixed ratio, and

5 means responsive to last said combined voltages for indicating thecorrect direction.

WILFRJD GORDON WHITE.

